In general, the process of electrophotography in the copiers and printers for Plain Paper Copy (PPC) in which a toner image formed on a photoconductor is transferred onto recording paper, involves first the formation of a latent electrostatic image on the photoconductor, subsequent development of this latent image by means of toner, transfer of this toner image onto a sheet to be fixed, for example, a sheet of paper, and then hot fixing the image with a heat roller. As it allows fixing under high temperature and high pressure, this process is fast and very good in the heating efficiency, and thus very good in the fixing efficiency. However, in such a heat-roll fixing method, while the heating efficiency is good, there is a problem (of the offset phenomenon) in which the contact between the heat roller surface and the toner in a molten state results in the toner adhering and being transferred onto the surface of the heat roller, which is re-transferred onto the next sheet to be fixed, thus making the sheet contaminated. A way of preventing such offset phenomenon includes, for example, application of an anti-offset liquid such as silicone oil onto the surface of the heat roller with clothes or paper. This method is very effective in view of preventing the offset phenomenon of the toner, but requires another extra device for supplying the anti-offset liquid, which leads to complication of the machine design, and it is criticized for causing an increase in the cost, increased maintenance of the machine, complication in the maintenance, contamination within the machine due to the evaporation of said liquid. Accordingly, development of a toner for high-speed machines (oil-less fixing method) used in a method that does not require such application of silicone oil (oil-less fixing method), is on demand.
Meanwhile, there is a demand for high-speed copier machines from the market and thus a toner that is compatible with a high-speed fixing roller, that is, capable of fixing by heating in a short time is demanded. This is considered as an important performance capable of coping with energy saving. In order for the fixing to be done in the shortest possible time, the toner needs to melt at low temperatures and to be highly fluent. For this, it is effective in general to lower the glass transition temperature (hereinafter, referred to as “Tg”) of the resin used in the toner; however, there is a problem of the toner undergoing so-called “blocking” during storage.
For the anti-offset processes in the development of a toner for the oil-less fixing method, toners using crosslinked polymers are often suggested. For example, Japanese Examined Patent Application Publication No. 60-36582 or the like discloses a method of using crosslinked polymer that is prepared by emulsion polymerization. In this case, the crosslinked polymer used contains 50 to 99% by mass of a gel component. As this gel component increases in amounts, the offset resistance becomes better, but the pulverizabiliy becomes poor. Meanwhile, as the crosslinked polymer component is reduced, pulverizability becomes better, but the offset resistance becomes poor, and it is difficult to satisfy both of the offset resistance and pulverizability. Further, in, this method (emulsion polymerization), it is necessary to use a dispersing agent or a dispersing aid in combination during the preparation of the crosslinked polymer, in order to stabilize the emulsion particles. Since such a dispersing agent is likely to absorb the moisture, it sometimes has adverse effects on the electrical properties, especially the charge stability. Therefore, it is necessary to remove the dispersing agents as much as possible after the preparation of the crosslinked polymer. Removal of them would be effective mainly with washing the polymer with water in an industrial scale; however, since the efficiency of washing is not always sufficient, in addition to the cost of installation, there are problems such as requiring huge quantities of washing water and increase in the cost of drainage.
In the publication of U.S. Pat. No. 4,966,829, disclosed is a good toner which contains a vinyl-based polymer having 0.1 to 60% by mass of the gel component, with the molecular weight of the main peak in GPC of the tetrahydrofuran (THF)-soluble component being 1000 to 25000 and the molecular weight of the sub-peak or the shoulder being 3000 to 150000. However, the process for preparing the polymer is the suspension polymerization method, and in this case, too, like the emulsion polymerization case, a dispersing agent or a dispersing aid is used in combination in the preparation, having entirely the same problems as in the case of the emulsion polymerization.
Meanwhile, in the solution polymerization method, there is a need to remove the solvent, after the completion of polymerization, by the so-called flash process in which the reaction solution is subjected to a reduced pressure, for example. In this case, since it is possible to seclude the low-volatile components such as unreacted residual monomers or decomposition products of the initiator, the impurities content is very low and charge-stable, and moreover, homogeneous resin can be obtained. For this reason, it is considered to be suitable as the method of preparing a binder resin for toner. However, preparation of a crosslinked polymer by the solution polymerization was associated with a problem of having the so-called Weisenberg phenomenon in which the reaction solution clings to the stirrer bar as the polymerization reaction proceeds.
Japanese Examined Patent Application Publication No. 60-38700 discloses a good toner binder prepared by heating and mixing a polymer (A) containing 3 to 40% by mass of a glycidyl group-containing monomer and a crosslinkable compound (B). However, since this toner comprised large quantities of residual epoxy radicals, testing for a long-term period caused generation of a toner of reverse charge, and there was a problem about durability.
As described in the above, an electrophotographic toner binder or toner that is excellent in the (low temperature) fixing property, offset resistance, blocking resistance, pulverizability, development durability or the like, which can cope with high-speed copier machines or energy saving, has not been known until now.